How do I test my stock alternator

[79josh81]

So my head lamp hasn’t worked since I bought the bike. The high beam indicator light on the tach also does not work. I’ve tested the head lamp and it does light up when connected directly to the battery. I went through the birds nest of wiring inside the head lamp and everything seems good.

I have a brand new battery that tests out at 12.4 V. I’m not super electrically inclined, but what I’ve gathered trying to search through other posts here is that the head lamp won’t come on if the alternator is shot?

I checked the battery voltage with the bike running and it’s the 12.4v that I mentioned at idle. When I throttle up to 3000 rpm it only goes up by like 0.1 or 0.2 v. I tried the RLU trick by connecting the wires and the headlamp still doesn’t work.

Is my alternator the cause of this problem? And besides eventually having a dead battery, what other indicator lights would be out as a result of a bad alternator?

FYI- My turn signals, tail light, and brake light all work.

 

[fredintoon]

Hi Josh,
if you don't see 13+ Volts at 3K rpm your battery is not being charged and your bike will only run until the battery Voltage drops to ~11volts.
Alternator failure is only one of the possible reasons for not charging.
Could be rectifier, regulator, wiring or any combination thereof that the PO neglect Gremlins could arrange.
On the bike alternator checks:-
Does the rotor become magnetic and suck a thin steel item (most folks use a feeler gauge) towards itself when the key is on?
Is the rotor partially shorted?
Check resistance between the slip rings. Should be 4+ Ohms. & between each slip ring and ground. Should have NO continuity.
Check the stator coils. The search button will tell you how. Then ask it how to check everything else.

 

[XSLeo]

Depends on the year of your bike whether the alternator controls the headlight.
On the early bike the alternator triggers a relay that controls the electric starter. On the later bikes there was second relay that turned on the headlight. This started around 78 with the Specials. On the 79's they even removed the headlight on/off switch. Not exactly sure on that though.
If you look up near the top of the page you will find a TECH button. This leads you to a list of topics. Under these main headings are subtopics. Just scroll down till you find the charging guide.
This guide takes you step by step through both the early and late charging systems. The main difference between the two systems is how the current flow through the rotor is controlled. On the early models the power to the rotor is controlled by the regulator.
On the later power ground is controlled by the regulator.
Leo

 

[mrtwowheel]

Fred's post...add, check brushes at rotor/stator and the wires to the brushes.

Scott

 

[79josh81]

Thanks for the input guys. I have a 1979 xs650 by the way. And my electric start works just fine.

 

[79josh81]

Hi Josh,
if you don't see 13+ Volts at 3K rpm your battery is not being charged and your bike will only run until the battery Voltage drops to ~11volts.
Alternator failure is only one of the possible reasons for not charging.
Could be rectifier, regulator, wiring or any combination thereof that the PO neglect Gremlins could arrange.
On the bike alternator checks:-
Does the rotor become magnetic and suck a thin steel item (most folks use a feeler gauge) towards itself when the key is on?
Is the rotor partially shorted?
Check resistance between the slip rings. Should be 4+ Ohms. & between each slip ring and ground. Should have NO continuity.
Check the stator coils. The search button will tell you how. Then ask it how to check everything else.

Well I have magnetic pull by the alternator cover when I turn the key on.

 

[mrtwowheel]

Well I have magnetic pull by the alternator cover when I turn the key on.

I don't even do that slap test, it's the other tests that matter.

Scott

 

[79josh81]

I don't even do that slap test, it's the other tests that matter.

Scott

Of course the easy test I understood doesn’t matter. Lol

 

[79josh81]

I’m going to sound like an idiot but I’m completely new to xs650’s and motorcycles in general...So just so I’m understanding this right, if I check my battery voltage with a multimeter when the bike is running, if the voltage doesn’t go up when I thottle up, something is not working right in the charging system correct?

 

[mrtwowheel]

Yes, should be somewhere above 12 volts at idle, at the battery. Should be somewhere above 13.5 volts at 3000 RPM at the battery, 14.2 volts is considered ideal at 3000 RPM. Tell us what you get.

Scott

 

[79josh81]

Yes, should be somewhere above 12 volts at idle, at the battery. Should be somewhere above 13.5 volts at 3000 RPM at the battery, 14.2 volts is considered ideal at 3000 RPM. Tell us what you get.

Scott

I got 12.4v no matter what rpm I’m running at. The resistance between the slip rings on the rotor is 4 ohm. If I did the slip ring to ground test right though, I got 4 ohm on the outside slip ring to ground and 1.1 ohm on the inside slip ring to ground.

 

[mrtwowheel]

I got 12.4v no matter what rpm I’m running at. The resistance between the slip rings on the rotor is 4 ohm. If I did the slip ring to ground test right though, I got 4 ohm on the outside slip ring to ground and 1.1 ohm on the inside slip ring to ground.

Unplug the wires that go to the brushes and retest the sliprings to ground.

Scott

 

[mrtwowheel]

I got 12.4v no matter what rpm I’m running at. The resistance between the slip rings on the rotor is 4 ohm. If I did the slip ring to ground test right though, I got 4 ohm on the outside slip ring to ground and 1.1 ohm on the inside slip ring to ground.

Yeah, good start. Need to find out if that short to ground is in the rotor or your wiring. Be sure that your probe is not touching the stator because that would make a connection to ground.

Scott

 

[79josh81]

Unplug the wires that go to the brushes and retest the sliprings to ground.

Scott

I work until Thursday now but I'll do that then and let you know. Thanks again.

 

[gggGary]

Basics first; check and clean battery cables. ground cable is often rotted at frame. corrosion etc. The 79 SPECIAL (had) a 4 fuse box fuses and fuse holders are often bad/rotted remove and check all of them.

From tech

 

[brianmac]

ps if u dont care about the stock appearance of the bike, replacing the fusebox is a good idea if your good at soldering wires.
walmart has these in line mini fuses that have led indicator lights when blown. youll have to solder and shrink em in, but they are also weatherproof. there are sizes, get the smallest , as all that matters is amperage, and wire size. i tucked them in under right side cover.


reference:

Blade fuses (also called spade or plug-in fuses), with a plastic body and two prongs that fit into sockets, are mostly used in automobiles.

Each fuse is printed with the rated current in amperes on the top.

These types of fuses come in six different physical dimensions:

• micro2.
• micro3.
• LP-mini (APS), also known as low-profile mini.Unofficially, the "low-profile mini" fuse is sometimes incorrectly called "micro" since the term means smaller than mini, but recently fuses using the micro name have been released.
• mini (APM / ATM).The mini fuses were developed in the 1990s.
• regular (APR / ATC / ATO / ATS), also known as standard.Regular blade-type fuses, also known as standard fuses, were developed in 1976 for low voltage use in motor vehicles.
• maxi (APX), heavy-duty.

 

[79josh81]

Basics first; check and clean battery cables. ground cable is often rotted at frame. corrosion etc. The 79 SPECIAL (had) a 4 fuse box fuses and fuse holders are often bad/rotted remove and check all of them.

From tech

I already removed the old fuse block and wired in a new one with brand new blade fuses. Thanks for the wiring diagram. This no charging sh*t makes me want to just spend the $300 on a PMA charging system but I’d like to not do that if I can find the issue. Where is the rectifier on a 79’ xs650? I know the voltage regulator is on the left side of the battery I think?

 

[Superjet]

Save your money and go thru the charging guide. PMA may not be your answer in the long run.

 

[brianmac]

https://www.partzilla.com/catalog/yamaha/motorcycle/1979/xs650-2f/electrical-2
learn this site
i think on the front of rear fender, i have like 4 or 5 of em, cheap . check harness pictures carefully.

 

[79josh81]

Save your money and go thru the charging guide. PMA may not be your answer in the long run.

Yeah I hear you. It’s just difficult learning some of this stuff considering 1 week ago was the first time I legitimately started learning to use a multimeter and I’ve never owned an xs650. I’m just glad this forum and lots of online xs650 sources are around.